Fluoropolymers, including homopolymers and copolymers of tetrafluoroethylene (TFE), collectively TFE polymers, are well known and widely used because of their unusual combination of chemical resistance, surface characteristics, dielectric properties, and high-temperature service capabilities. Depending on chemical composition, i.e., the identity and proportion of monomer units in the polymer, a fluoropolymer may be partially crystalline or amorphous, plastic or elastomeric. Amorphous dipolymers of TFE and perfluorinated alkyl vinyl ethers are known to be elastomeric, with low glass transition temperatures (T.sub.g), usually less than 10.degree. C. desirably less than 0.degree. C. More rigid amorphous fluoropolymers, having T.sub.g at about room temperature or higher, are desired.
Certain copolymers of TFE and hexafluoropropylene (HFP) are known. For example, Bro & Sandt in U.S. Pat. No. 2,946,763 disclose TFE/HFP copolymers with HFP content reflected by a specific infrared ratio, herein called HFP index or HFPI, in the range 1.5 to 6. They use a multiplier of 4.5 to convert HFPI to HFP content in wt %. There are frequent references in the literature to TFE/HFP copolymers having HFP content of from 6.75 to 27 wt %, apparently following the disclosure of the 1.5 to 6 HFPI range and 4.5 multiplier by Bro & Sandt. Recent workers in the field have refined their compositional calibrations of HFPI and now generally use multipliers in the range of 3.0-3.2 to convert HFPI to HFP content in wt %.
Morgan in U.S. Pat. No. 5,266,639 discloses partially crystalline TFE/HFP copolymers having HFPI of 6.4 to about 9, made by a semi-batch dispersion polymerization process. Morgan teaches that a high concentration of surfactant can impede iolation of the copolymer resin and that surfactant concentration should be less than 0.4 wt %, preferably less than 0.2 wt %, based on the aqueous medium. Surfactant concentrations exemplified were in the preferred range.
Khan in U.S. Pat. No. 4,381,384 discloses a continuous polymerization process for TFE polymers including copolymers containing up to 20 mol % of a variety of comonomers. For TFE/HFP copolymers, a multiplier of 2.1 was used to convert HFPI to HFP content in mol %, so that the limit of 20 tool% would correspond to an HFPI of 9.5. In the examples for TFE/HFP copolymers, HFP content of 5.4 mol % (HFPI of 2.57) was the highest level actually achieved. The continuous process of Khan in U.S. Pat. No. 4,381,384 suffers from some disavantages, including a very high surfactant concentration that must be used to approach stable reactor operation and to enable discharge of the reaction mass without coagulation of the polymer in or in passing through the let-down valve. This high surfactant concentration can, in turn, make it extremely difficult to isolate the polymer from the dispersion, and can be undesirable in the isolated product. Space-time yield for Khan's low-HFP copolymer was of the order of only 0.1 kg/L.hr.
Khan discloses that TFE/HFP dipolymers having HFP content reflected by HFPI=9.5 are partially crystalline, though HFP incorporation did not approach this level. Morgan discloses that such polymers having HFPI=9 are crystalline, and did not achieve higher levels of HFP incorporation.
Eleuterio in U.S. Pat. No. 3,062,793 describes amorphous copolymers. Such copolymers having HFPI values of about 17 and 22 are exemplified. Eleuterio also discloses a partially crystalline TFE/HFP copolymer having HFPI=10.5 and 15% crystallinity. This level of crystallinity is surprisingly high for the HFPI value, which corresponds to HFP content of about 25 mol % as estimated from examples to follow, especially after melt pressing of the sample. However, Eleuterio's method of synthesis, with all of the TFE and HFP monomers present at the start of the polymerization reaction, would be expected to produce very non-uniform copolymers because HFP is much less reactive than TFE.
Although it is well-established commercial practice to make winter shipments of aqueous dispersions of TFE homopolymers and copolymers in insulated and/or heated trucks to prevent freeze damage, typically irreversible coagulation, no commercial utilization of a freezing process for isolation of TFE polymer resin from its polymerization medium is known. A freeze drying process, involving sublimation of the water from the frozen coagulate or dispersion, for fibrillatible, non-melt fabricable TFE polymer usually supplied as an agglomerate of dispersion particles (often called fine powder) is disclosed by Ocone in U.S. Pat. No. 3,692,759. Furuya & Motoo in U.S. Pat. No. 5,816,431 disclose a process for preparing a dispersion of raw materials for reaction layers of a gas permeable electrode comprising mixing carbon black, polytetrafluoroethylene, water and surfactant, freezing this mixture and then thawing out the mixture.